package org.dizena.utils;

public class IdGen
{

    private long workerId;
    private long datacenterId;
    private long sequence = 0L;
    private long workerIdBits = 5L;
    private long datacenterIdBits = 5L;
    /**
     * 最大支持机器节点数0~31，一共32个
     */
    private long maxWorkerId = -1L ^ (-1L << workerIdBits);
    private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
    private long sequenceBits = 12L;
    private long workerIdShift = sequenceBits;
    private long datacenterIdShift = sequenceBits + workerIdBits;
    private long timestampLeftShift = datacenterIdShift + datacenterIdBits;
    private long sequenceMask = -1L ^ (-1L << sequenceBits);
    private long lastTimestamp = -1L;

    private static class IdGenHolder
    {
        private static final IdGen instance = new IdGen();
    }

    public static IdGen get()
    {
        return IdGenHolder.instance;
    }

    public IdGen()
    {
        this(0L, 0L);
    }

    public IdGen(long workerId, long datacenterId)
    {
        if (workerId > maxWorkerId || workerId < 0)
        {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
        }
        if (datacenterId > maxDatacenterId || datacenterId < 0)
        {
            throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
        }
        this.workerId = workerId;
        this.datacenterId = datacenterId;
    }

    public synchronized String nextSid()
    {
        return nextId() + "";
    }

    public synchronized String nextIdS()
    {
        return nextId() + "";
    }

    public synchronized long nextId()
    {
        //获取当前毫秒数
        long timestamp = timeGen();
        // 如果服务器时间有问题(时钟后退) 报错。
        if (timestamp < lastTimestamp)
        {
            throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }
        // 如果上次生成时间和当前时间相同,在同一毫秒内
        if (lastTimestamp == timestamp)
        {
            // sequence自增，因为sequence只有12bit，所以和sequenceMask相与一下，去掉高位
            sequence = (sequence + 1) & sequenceMask;
            // 判断是否溢出,也就是每毫秒内超过4095，当为4096时，与sequenceMask相与，sequence就等于0
            if (sequence == 0)
            {
                // 自旋等待到下一毫秒
                timestamp = tilNextMillis(lastTimestamp);
            }
        }
        else
        {
            // 如果和上次生成时间不同,重置sequence，就是下一毫秒开始，sequence计数重新从0开始累加
            sequence = 0L;
        }
        lastTimestamp = timestamp;
        // 最后按照规则拼出ID。
        // 000000000000000000000000000000000000000000 00000 00000 000000000000

        // time datacenterId workerId sequence
        long twepoch = 1288834974657L;
        return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift) | (workerId << workerIdShift) | sequence;
    }

    protected long tilNextMillis(long lastTimestamp)
    {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp)
        {
            timestamp = timeGen();
        }
        return timestamp;
    }

    protected long timeGen()
    {
        return System.currentTimeMillis();
    }

}
